Rosin esters comprising polyhydric alcohols are well known in the prior art. Woodruff, U.S. Pat. No. 2,424,424, discloses high molecular weight esters comprising rosin and pentaerythritol. Johnson, U.S. Pat. No. 4,758,379, discloses a method of preparing polyol esters of rosin comprising carrying out the esterification in an equivalent excess proportion of the rosin. Further, Maeda et al., U.S. Pat. No. 4,847,010, disclose a process for preparing a rosin ester which comprises purifying a rosin and/or a disproportionated rosin, esterifying the purified rosin and/or purified disproportionated rosin with an alcohol and subjecting the esterified rosin and/or esterified disproportionated rosin to hydrogenating.
Catalysts are often used to accelerate the reaction rate of the esterification of rosin with a polyhydric alcohol, as well as to provide color improvements. In Duncan, et al., U.S. Pat. No. 4,548,746, phosphinic acid is used in very small quantities to act as a catalyst for esterification of rosin with pentaerythritol. In U.S. Pat. No. 4,650,607, Lampo et al., the catalytic combination of phosphinic acid and a phenol sulfide compound is disclosed.
Acid-modified polyhydric alcohol rosin esters are also known in the prior art. Savageau et al., U.S. Pat. No. 3,772,171, disclose a maleic acid-modified pentaerythritol rosin ester for use in quick-setting ink formulations. McInnes et al., U.S. Pat. No. 3,874,898, disclose isophthalic-modified pentaerythritol rosin esters which are used in the formulation of paste printing ink vehicles. A binder for toner preparation comprising as the main constituent thereof a polyester composed of a polybasic acid component and a polyhydric alcohol component with a rosin-modified polyhydric alcohol introduced as part of the polyhydric alcohol component is disclosed in Matsumura et al., U.S. Pat. No. 4,981,939. Polybasic acids disclosed therein for use in those binders include terephthalic acid and isophthalic acid.
There is a long felt need to develop tackifiers for use in hot melt adhesive compositions. While the existence of polyhydric alcohol rosin esters, including acid-modified pentaerythritol rosin esters, is well documented in the prior art, none of the rosin esters discussed heretofore have been employed as tackifiers in hot melt adhesive compositions. This is due in part to compatibility problems between the rosin esters described heretofore and polymers or copolymers used in hot melt adhesive formulations.
In order for the rosin ester tackifiers to be used in hot melt adhesive compositions, it is necessary that the softening point of the rosin ester tackifier be sufficiently high for use in the hot melt compositions. Additionally, the rosin ester tackifier must be relatively compatible with the polymers or copolymers used in the hot melt adhesives. That is, the hot melt adhesive compositions containing the rosin ester tackifier should be relatively clear, i.e. not too hazy or opaque, and viscosity stable, i.e. not susceptible to excessive viscosity increase over time.
In U.S. Pat. No. 4,725,384, Du Vernet, rosin esters prepared by reacting a rosin and a polyhydric alcohol in the presence of a catalytic combination of phosphinic acid and a phenol sulfide compound are further treated with a magnesium salt of an organic acid to improve the clarity and viscosity stability of hot melt adhesives using those rosin esters as tackifiers.
One approach to increasing the softening point of the rosin ester tackifier is described in Matsuo, et al., U.S. Pat. No. 4,302,371. In Matsuo, a process for preparing a stabilized rosin ester having a higher softening point is disclosed which comprises subjecting a rosin to disproportionation and purification, and esterifying the resulting purified, disproportionated rosin with a tri- or more valent polyhydric alcohol, to give a rosin having a ring and ball softening point of 65.degree. C. to 140.degree. C. Matsuo, et al. found that, even by preparing a rosin ester by first purifying a rosin as a starting material to remove the higher molecular materials and unsaponifiable materials and then esterifying the purified rosin with the polyhydric alcohol, the properties of the rosin ester thus obtained were not sufficiently improved to be used as hot melt adhesives.
In order to prepare rosin esters having sufficient properties for use in hot melt adhesives, Matsuo et al. found that it was necessary to first disproportionate the rosin as a starting material, then purify the disproportionated rosin to remove high molecular materials and unsaponifiable materials, and finally esterify the purified, disproportionated rosin with a polyhydric alcohol. The methods of Matsuo are uneconomical and undesirable in that the disproportionation and purification methods, such as distillation under reduced pressure, crystallization and extraction, are time consuming, require additional expensive apparatus and restrict the supply of rosin used in the esterification process for preparation of the rosin esters. Therefore, the hot melt adhesives formulated with the rosin ester tackifier will likewise be economically undesirable.
There is a need then for hot melt adhesive compositions which may be used at elevated temperatures. There is further a need for hot melt adhesive compositions which contain rosin ester tackifiers having elevated softening points. The tackifiers must be stable and compatible with polymers or copolymers used in the preparation of the hot melt adhesive compositions, and must be prepared by economically desirable methods.
The Applicants of the present invention have surprisingly discovered that hot melt adhesive compositions can be prepared using acid-modified polyhydric alcohol rosin ester tackifiers having elevated softening points over comparable unmodified polyhydric alcohol rosin esters. The acid-modified tackifiers are prepared by methods which eliminate the need for the complicated and economically undesirable disproportionation/purification methods.